The spatial density and fraction of FGs were estimated in different studies, both theoretically and observationally. The agreement on the spatial density is quite good, with estimations in the range \(\sim 1-3 \times 10^{-6} \,h_{50}^{3}\)  Mpc\(^{-3}\)  [1]}, [2]}, [3]}. On the other hand, the fraction of FGs with respect to the total amount of clusters and groups is more debated. A comparison between different works can be found in table 1 of [4]}: the estimated fractions varies between 1% and 40%, with a large scatter, and a mean value can be found at about \(\sim 10-15\%\) .
